The present invention relates generally to laser resonators and associated methods and, more particularly, to chemical laser resonators and associated methods.
Chemical lasers, such as chemical oxygen iodine laser (COIL) devices, generate output signals having relatively high intensity. As such, chemical lasers are used in a variety of applications including high energy laser (HEL) weapons and the like.
For purposes of explanation, a conventional chemical laser 10 is depicted in FIG. 1. A chemical laser includes a laser cavity 12 having opposed upstream and downstream ends 14, 16. A lasing medium flows through the laser cavity in a predetermined flow direction from the upstream end of the laser cavity to the downstream end of the laser cavity. While chemical lasers can include different lasing mediums, a COIL device generally includes a lasing medium containing the species O2(1xcex94) (electronically-excited oxygen) and Ix (electronically-excited iodine atoms). As shown in FIG. 1, the laser cavity typically includes an upstream end defined by a nozzle 18 through which the lasing medium enters the laser cavity and a downstream end through which the lasing medium exits the laser cavity after extraction of some of the medium energy in the form of a laser beam.
FIG. 1 also depicts the resonator structure that cooperates with the laser cavity 12 for producing the output signals having a relatively high intensity level. The resonator of FIG. 1 is a negative branch standing wave resonator in which the laser beam makes a single pass through a single laser cavity. In this regard, only the collimated passes of the laser beam through the laser cavity are counted with the laser beam also assumed to traverse the laser cavity as an expanding beam, as shown in FIG. 1. The laser resonator is comprised of several optical components, including a scraper 20 for outcoupling a substantial portion of the laser beam, thereby providing the output signals. The laser resonator also includes a secondary mirror 22 positioned rearwardly of the scraper relative to the laser cavity for reflecting a portion of the laser beam back through the laser cavity. As depicted in FIG. 1, the secondary mirror redirects a portion of the laser beam, typically a center portion of the laser beam, in such a manner that the laser beam again expands to its full size prior to being collimated. The laser resonator also includes a primary mirror 24 positioned opposite the scraper and the secondary mirror relative to the laser cavity for collimating the expanding laser beam and for redirecting the collimated laser beam back through the laser cavity.
Since the secondary mirror 22 is a concave mirror positioned rearward of the scraper 20 relative to the laser cavity 12, the portion of the laser beam redirected by the secondary mirror must pass through focus prior to expanding and being collimated by the primary mirror 24. As such, the laser resonator of FIG. 1 is referred to as a negative branch standing wave resonator. Alternatively, the secondary mirror could be a convex mirror positioned proximate to the scraper or rearwardly of the scraper relative to the laser cavity for redirecting a portion of the laser beam back through the laser cavity in such a manner that the redirected portion of the laser beam expands while passing through the laser cavity without passing through focus. As known to those skilled in the art, this alternative type of resonator structure is a positive branch standing wave resonator.
Since the chemicals that produce the gain and/or power are progressively consumed as the laser beam is generated, the gain or amplification provided by the chemical laser progressively drops as the distance of the laser beam from the nozzle 18 increases. By way of illustration, FIG. 2a depicts the gain provided by the laser cavity 12 as a function of the distance from the nozzle with signals that traverse the laser cavity at distances further from the nozzle having lower gain. As such, the resulting outcoupled beam has a strongly peaked (xe2x80x9csugar scoopedxe2x80x9d) intensity profile as depicted in FIG. 2b. As will be noted, the intensity profile has a gap in the center portion thereof due to beam obscuration caused by the redirection of a portion of the laser beam back through the laser cavity by the secondary mirror 22. As a result of the peaked intensity profile, the chemical laser 10 of FIG. 1 does not generate the laser beam as efficiently as desired. Additionally, since the intensity of the laser beam varies across the width of the laser beam, the propagation of the outcoupled laser beam is degraded. In this regard, those portions of the laser beam having a greater intensity will heat the air or other atmosphere through which the laser beam is propagating to a greater degree than those portions of laser beam having a lower intensity. Thus, different portions of the laser beam will be diffracted in unequal amounts by the differently heated air. Moreover, in order to ensure that the average intensity of the laser beam is greater than a predetermined intensity level, that portion of the laser beam having the greatest intensity may have an excessively high intensity which may cause coating damage to the laser optics.
In order to avoid at least some of the shortcomings of the chemical laser 10 depicted in FIG. 1, other types of chemical lasers have been designed. As depicted in FIG. 3, for example, another conventional chemical laser 30 is designed such that the laser beam passes through the laser cavity 32 twice as a collimated laser beam. In this regard, the laser resonator of the embodiment of FIG. 3 includes several additional mirrors to invert and translate the laser beam. As described above in conjunction with the embodiment of FIG. 1, the gain provided by the lasing medium diminishes in the downstream direction since the chemicals that produce the gain and/or power are progressively consumed as the laser beam is generated. By including mirrors to invert the laser beam following each pass through the laser cavity, one edge of the laser beam is not always nearest the upstream portion of the laser cavity and the other edge of the laser beam is not always nearest the downstream portion of the laser cavity. Instead, each edge of the laser beam is located nearest both the upstream and downstream portions of the laser cavity during different passes through the laser cavity. As such, the intensity profile is no longer xe2x80x9csugar scoopedxe2x80x9d. In this regard, FIG. 4 provides a graph depicting the intensity of the outcoupled beam as a function of the distance across the beam which illustrates that the opposed edges of the laser beam have the greatest intensity and the center portion of the laser beam has the lowest intensity. While the intensity profile is more uniform and the chemical laser of FIG. 3 generally has improved extraction efficiency relative to the chemical laser of FIG. 1, the chemical laser of FIG. 3 disadvantageously requires additional powered optics, i.e., those optical elements, such as mirrors, that reflect or otherwise redirect a collimated laser beam. In addition, the length of the laser resonator can become relatively long since considerable spacing is typically required between the laser cavity and the optical elements.
Another conventional chemical laser 40 that is designed to create a more uniform laser beam intensity profile is depicted in FIG. 5. In this embodiment, the chemical laser includes first and second laser cavities 42, 44 to provide smoothing of the beam profile. As shown, the chemical laser of this embodiment is similar to the embodiment depicted in FIG. 1 with the exception that the laser beam passes through not one, but two laser cavities that are positioned side-by-side. By positioning the laser cavities such that the direction of flow of the lasing medium through one laser cavity 18 is opposite the direction of flow of the lasing medium through the other laser cavity, a gain profile is produced that is slightly U-shaped, but that does not drop off sharply from either edge of the laser beam. In this regard, FIG. 6 depicts the gain associated with each laser cavity (designated Nozzle A and Nozzle B) as well as the total gain of the chemical laser as a function of the distance from Nozzle A. As such, the intensity profile for a laser beam having passed through the chemical laser of FIG. 5 as a function of the distance across the beam is the same as the intensity profile depicted in FIG. 4 in conjunction with the embodiment of the chemical laser of FIG. 3. In this regard, the edges of the outcoupled laser beam each have a greater intensity than a center portion of the laser beam. While the chemical laser of FIG. 5 has a more uniform laser beam intensity profile than that provided by the chemical laser of FIG. 1, the chemical laser of FIG. 5 is substantially more complex and expensive since an entirely separate second laser cavity has been added.
While a variety of chemical lasers have therefore been developed, conventional chemical lasers still suffer from shortcomings. In this regard, the chemical lasers that have the least complex design generally produce laser beams having a strongly peaked (xe2x80x9csugar scoopedxe2x80x9d) intensity profile. Not only is the generation of such strongly peaked intensity profiles relatively inefficient, but the propagation of the laser beam may be degraded due to differential heating of the air or other atmosphere through which the laser beam propagates. In addition, in embodiments in which a relatively high average intensity is desired, the portion of the laser beam having the highest intensity may be of a sufficiently high intensity to cause coating damage to the laser optics. While other chemical lasers have been designed that generate a more uniform intensity profile, these other chemical lasers generally have a substantially more complex design by requiring either multiple passes through a single laser cavity or a single pass through multiple laser cavities. As such, it would be desirable to develop a chemical laser having a relatively simple design that produces a laser beam having a relatively uniform intensity profile.
A laser resonator and an associated method are therefore provided according to the present invention which produce a laser beam having a relatively uniform intensity profile following a single pass through a single laser cavity. As a result, the laser resonator and associated method of the present invention are substantially less complex than at least some of the conventional chemical laser designs. By producing a laser output having a more uniform intensity profile, however, the laser resonator and associated method of the present invention can efficiently generate a laser beam in a manner that should not cause coating damage to the laser optics and should permit propagation of the laser beam in a relatively uniform manner.
The laser resonator includes a laser cavity having opposed upstream and downstream ends. As such, the laser cavity is adapted to support flow of a chemical medium in a predetermined flow direction from the upstream end of the laser cavity to the downstream end of the laser cavity. The laser resonator also includes a first optical element, typically termed a secondary mirror, for directing an expanding laser beam across the cavity. The expanding laser beam has opposed first and second opposed edges and the expanding laser beam is directed across the laser cavity in such a manner that the first edge of the expanding laser beam is nearer the upstream end of the laser cavity than the second edge of the expanding laser beam. The laser resonator also includes a second optical element disposed opposite the first optical element relative to the laser cavity. Typically, the second optical element is a set of primary mirrors and, more particularly, a pair of primary mirrors for both collimating and spatially inverting the laser beam relative to the predetermined flow direction of the chemical medium through the laser cavity. Once the expanded laser beam has been collimated and spatially inverted, the laser beam is redirected across the laser cavity. As such, the first edge of the redirected laser beam is now downstream of the second edge of the redirected laser beam.
The laser resonator can also include a third optical element that receives the redirected laser beam following its propagation across the laser cavity and that forms an output laser beam following a single pass of the collimated laser beam across the laser cavity. Typically, the third optical element is positioned proximate the first optic element and can be a scraper mirror.
According to the present invention, the portions of the expanding laser beam proximate the first edge are more greatly amplified than portions of expanding laser beam proximate the second edge while the expanding laser beam initially propagates across the laser cavity. By spatially inverting the laser beam, however, the portions of the redirected laser beam proximate the second edge are more greatly amplified than the portions of the redirected laser beam proximate the first edge while the redirected laser beam propagates across the laser cavity. As such, the intensity profile of the output laser beam is relatively uniform. In addition, the design of the laser resonator is much less complex than at least some conventional chemical lasers.